1. Field of Invention
This invention relates to a method for producing nickel nanoparticles.
2. Description of Related Art
Since nickel nanoparticle is cheaper and also more chemically stable than noble metal nanoparticles such as silver nanoparticle, its application to catalysts, magnetic materials, electrodes of multilayer ceramic capacitors and so on is expected. Nickel nanoparticles are conventionally prepared through a solid-phase or liquid-phase reaction. Known solid-phase reactions include chemical vapor deposition using nickel chloride, thermolysis of nickel formate, and so on. Known liquid-phase reactions include: direct reduction of a nickel salt such as nickel chloride by a strong reductant such as sodium borohydride, thermolysis of a precursor such as [Ni(H2NNH2)2]SO4·2H2O that is formed by adding a reductant such as hydrazine in presence of NaOH, hydrothermal synthesis by loading a nickel salt such as nickel chloride or a nickel complex containing organic ligands together with a solvent in a pressurized container, and so on.
In order to supply nickel nanoparticles suitable for the above applications such as catalysts, magnetic materials and electrodes, it is necessary to control the particle sizes of the nickel nanoparticles to be uniform and less than 150 nm approximately.
However, in the method utilizing chemical vapor deposition among the methods utilizing solid-phase reactions, the particle is increased in size in an order of sub-micron to micron. Moreover, in the method utilizing thermolysis, the particles aggregate due to the high reaction temperature. Further, as compare with those utilizing liquid-phase reactions, the methods utilizing the solid-phase reactions have higher production costs.
On the other hand, in a method of liquid-phase reaction using a strong reductant, nickel is immediately reduced so that controlling the reaction to produce particles with desired particle sizes is difficult. Moreover, in the method utilizing a precursor, the precursor tends to gelate and renders the subsequent reduction reaction inhomogeneous, while in the hydrothermal synthesis method, the reaction temperature is high. Hence, aggregation cannot be prevented in any of these cases.
With respect to the liquid-phase reaction techniques, a method for producing Ni nanoparticles has been disclosed (in Patent Document 1), including a step of preparing a mixed solution by adding a reductant, a dispersant and a nickel salt in a polyol solution, a step of stirring and heating the mixed solution, and a step of reacting the same to form nickel nanoparticles. However, with this producing method, nickel nanoparticles with a high dispersity are not necessarily obtained well. Moreover, if there is residual reductant as an impurity in the nickel nanoparticles, depending on the application of the nickel nanoparticles, there are cases where the product quality is affected.
Moreover, a technique for producing Ni nanoparticles, which includes mixing a nickel precursor material, an organic amine and a reductant and then heating the same, has been disclosed (in Patent Document 2). However, when a strong reductant is used in the method, the reduction reaction is difficult to control, and Ni-nanoparticles with a high dispersity are not necessarily obtained well. On the other hand, when a weak reductant is used, high-temperature heating is required to reduce to metallic nickel that has a negative redox potential, and accompanying reaction control is demanded.